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Full text of "Handbook Of Chemical Engineering - I"

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REFRACTORIES                   '                      507
Foote found the total thermal expansion of a silica brick to be 0.7 per cent of the original length at 600°C., 0.9 per cent at 90° and 1.1 at 1,200°C.
Silica brick do not resist sudden temperature changes well but the harder they have been fired the better they behave in this respect. Refractories containing more tri-dymite should be expected to stand up better. As a class silica refractories show a far greater tendency to crack or spall upon chilling or rapid heating than clay materials.
Their resistance to slagging action as a whole is not great, owing to their high porosity and acid character. The compressive strength of silica bricks may vary from "2,000 to 5,000 Ib. per sq. in." and their modulus of rupture from 500 to 1,500. A modulus of 500 Ib. per square inch should be specified for the well-burnt product. Silica brick are used largely in the construction of by-product coke ovens, gas plants, the arches of open-hearth and glass furnaces.
Clay-bonded Silica Refractories.-—Ground quartzite rock may be bonded also with clay in place of lime and refractories thus made are frequently known as "quartzite." The amount of plastic fireclay required to bond together the quartzite grains is considerably greater than that of lime used for the regular Hilica brick, with the result that the softening point is depressed very decidedly according to Seger's curve and especially is this the case if the clay contains fluxing impurities. Thus, considering the melting point of pure silica to be 1,710°C., a rapid drop in fusibility with increasing clay content to that of the eutectic mixture is to be expected and below this point proportionately with the content of basic fluxcH present. The eutectic mixture consists of approximately 80 per cent of free silica and 20 of anhydrous clay. The physical properties of this type of refractories have not been established with any degree of accuracy. Mixtures of giound quarUite or ground silica brick and 15 to 35 per cent of clay are commonly used as mortar for laying up silica bricks.
AH a cljtHH the clay-bonded silica refractories are therefore inferior to the lime-bomled material JIB regards refractoriness and are not superior to the latter in strength, ability to resist, load conditions, resistance to sudden temperature changes, etc., but they usually show a lower after-expansion and sometimes no changes upon reheating. Their use has been principally in the construction of beehive coke ovens, brick kilns, annealing ovens, etc. They should not be made part of large structures like by-product, coke ovens us their expansion is too irregular.
Aluminous Refractories.—It can be readily seen that alumina itself, owing to its hi^h fusion point, 2,050°C., is inherently an excellent refractory. The only difficulty in the way of utilizing it is its large contraction upon heating.
Thin makes it necessary to condense it through the use of the electric furnace 1 >y Hint crintf or fusion. The point at which alumina, introduced as bauxite or diaspore, condenses to minimum volume or approximately to this point is necessarily a function of the, content, of impurities, principally of silica and iron oxide. The purer the raw materinl the. higher must be the sintering or fusion temperature. The fusion diagram of the system Si()2.Al./)a as given by Rankin and Wright, Geophysical Laboratory, is shown in KiR. 10. Refractories consisting principally of beta alumina may be prepared cither from the calcined material alone through fine grinding of part of it or by admixture, with Home plastic clay as a bond. It is evident that the introduction of the latter will at- once lower the fusion point according to the part ED of the fusion curve.
The. fusion temperature of the eutectic between A1203 and the next compound, AlaOruSiOji, is 1,S1()°0. The use of less refractory clay will lower this point still more and if t he amount added is at all large, it will be the determining factor in the deforma-tion under the effect of heat. Aluminous refractories should therefore contain but